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IC 9 ° 13 



Bureau of Mines Information Circular/1985 



Overcoring Equipment and Techniques 
Used in Rock Stress Determination 
(An Update of IC 8618) 



By David L. Bickel 




UNITED STATES DEPARTMENT OF THE INTERIOR 



75! 

AMINES 75TH A^ 



Information Circular 9013 



Overcoring Equipment and Techniques 
Used in Rock Stress Determination 
(An Update of IC 8618) 



By David L. Bickel 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 







Library of Congress Cataloging in Publication Data: 



Bickel, David L 

Overcoring equipment and techniques used in rock stress determina- 
tion (an update of IC 8618). 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 9013) 

Bibliography: p. 21. 
Supt. of Docs, no.: I 28.27:9013. 

1. Rocks— Testing— Equipment and supplies. 2. Rock mechanics. I. 
Title. II. Series: Information circular (United States. Bureau of Mines) ; 

9013. 

TN295.U4 [TA706.5] 622s [624. 1*5132] 84-600280 



CONTENTS 



Page 



\^Abstract 1 

Introduction 2 

KJ Acknowledgment 2 

Drilling equipment 2 

Instrumentation 7 

Site selection 9 

Overcoring procedure 11 

Procedure for operating the three-component borehole gauge 16 

Procedure for biaxial testing of the overcore to determine its anisotropic 

parameters 17 

Borehole gauge calibration 18 

Calibration data analysis 19 

Identification of source of borehole gauge malfunction 20 

Lack, of balance on one or more indicators 20 

Insensitivity of one or more elements on indicator 20 

Lack of component balance 20 

Sensitivity of indicators when touched 20 

References 21 

Appendix. — Equipment diagrams 22 

ILLUSTRATIONS 

1. Type CP-65 air drill 3 

2. EX-size bit with core spring, reamer, and 2-ft EWX core barrel 3 

3. Stabilizer on 2-ft section of EW drill rod, stabilizer on 1-ft section 

of BX wire line drill rod, and 2-ft piece of BX wire line drill rod.... 4 

4. Water swivel with solid plug and plug used during overcoring 4 

5. Core barrel, 6 in. in diam by 27 in long (referred to as a 2-ft core 

barrel), and disassembled expander head 5 

6. Expander head assembled (adapted for BX wire line drill rod) and 6-in- 

diam core barrel 5 

7. Starting the 6-in-diam overcoring hole 5 

8. Starter barrel, 6 in. in diam by 1 ft long, with pilot and expander 

head adapted for EW drill rod 6 

9 . Centering stabilizer 6 

10. Starting the EX hole in the bottom of the 6-in-diam hole 6 

11. Core breaker, core shovel, and core puller 7 

12. Retrieval tool used in place of core puller to retrieve 6-in-diam core 

from a vertical hole that has an EX hole through the core 7 

13. Special pliers, Bureau of Mines three-component borehole gauge, piston, 

disassembled piston and washer, and transducer with nut 8 

14. Strain indicator with calibration device and switching unit 8 

15. Placement and retrieval tool 9 

16. Biaxial chamber and pump 10 

17. Drill hole configurations 10 

18. Setup for three-component (3-D) borehole gauge emplacement 11 

19. Wire hookup to strain indicators 12 

20. Field data sheet 13 

21. Positioning three-component (3-D) borehole gauge with placement rod 

through the 6-in-diam core barrel and drill rod 14 

k A-l . Three-component (3-D) borehole gauge 22 

l v A-2. Borehole gauge placing and retrieving tool 23 



ILLUSTRATIONS— Continued 



Page 



A-3. Calibration jig and transducer testing jig 24 

A-4. Biaxial pressure chamber 25 

A-5. Borehole gauge placement tool for biaxial chamber 26 

A-6. Triaxial pressure chamber 27 





UNIT OF MEASURE ABBREVIATIONS 


USED IN 


THIS REPORT 


ft 


foot 




pet 


percent 


hp 


horsepower 




psi 


pound per square inch 


in 


inch 




rpi 


revolution per inch 


min 


minute 




rpm 


revolution per minute 


pin 


microinch 




s 


second 


yin/in 


microinch per 


inch 







OVERCORING EQUIPMENT AND TECHNIQUES USED 

IN ROCK STRESS DETERMINATION 

(AN UPDATE OF IC 8618) 

By David L. Bickel 1 



ABSTRACT 

Stress-relief techniques and instrumentation have been developed 
through many years of research in the Bureau of Mines and successfully 
used to determine the in situ state of stress in rock. This report de- 
scribes the Bureau's three-component borehole deformation gauge and the 
drilling equipment and accessories used with it. Operation and calibra- 
tion procedures for the deformation gauge are discussed in detail, along 
with site selection and overcoring procedures. Information and refer- 
ences included in this publication provide a guide to the Bureau's over- 
coring method for determining rock stress. 



'Physical scientist, Denver Research Center, Bureau of Mines, Denver, CO. 



INTRODUCTION 



This publication updates some of the 
overcoring procedures described in IC 
8618 06). 2 It also includes the field 
biaxial testing procedure used for deter- 
mining the modulus of elasticity, as well 
as additional references not previously 
available. 

The Bureau of Mines three-component 
borehole deformation gauge (9^, which 
measures three diameters 60° apart and 
has all sensing elements in the same 
plane, is designed to measure diametral 
deformations of a 1-1/2-in borehole dur- 
ing stress relief by overcoring. The 
process essentially consists of (1) 
drilling a 1-1/2-in-diam gauge hole 
(pilot hole) with a diamond bit and ream- 
er, (2) positioning the gauge in the 
gauge hole, and (3) drilling over the 
gauge with a 6-in-diam thin-walled dia- 
mond masonry bit. The 6-in core is cut 
in one continuous run, and the deforma- 
tion readings are taken at the start, 
during, and at the end of the run. After 
overcoring is completed, the gauge is re- 
moved and the core is freed from the 
bottom of the hole and retrieved. The 



core orientation and the position of the 
gauge are marked on the core. 

The marked core is then tested in a bi- 
axial chamber (4) to determine the physi- 
cal properties of the rock. The physical 
properties and the deformation measure- 
ments from each overcore are used to cal- 
culate the stress distribution in the 
plane normal to the axis of the borehole. 
The core can then be prepared and tested 
in a triaxial chamber (10) at the same 
stress level it experienced in situ 
(which is not always possible during bi- 
axial testing) to determine Poisson's ra- 
tio and a more accurate modulus of elas- 
ticity. The effects of anisotropy can be 
included in these calculations (1). If 
borehole deformation measurements are ob- 
tained from at least three nonparallel 
holes, the three-dimensional representa- 
tion of the average ground stress compo- 
nents can be determined (11) . A least 
squares method of calculating the average 
rock stress components, from more than 
three diametral deformation measurements 
in a borehole has been developed (3). 



ACKNOWLEDGMENT 



Acknowledgment is extended to Verne E. 
Hooker, senior author of the previous 
edition of this report. Mr. Hooker was 



with the Bureau's Denver 
before his retirement. 



Research Center 



DRILLING EQUIPMENT 



The following drilling equipment should 
be employed for in situ stress measure- 
ments: 

1. A drill with a chuck speed ranging 
down to 120 rpm and a penetration rate of 
1/2 in per 30 to 50 s using a 6-in-diam 
overcoring bit; the chuck and quill must 
be large enough to accomodate EW 3 drill 
rod (fig. 1). Since drills do not 

— 
Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 

J E, EW, B, and N denote diameter or 
size. 



normally have chuck speeds as low as 120 
rpm, a change in gear ratio may be neces- 
sary. For a typical 20-hp air drill, the 
recommended rotation speed is to 1,000 
rpm and the feed ratios are 200, 300, 
500, and 800 rpi. 

2. Two EWX 4 double-tube swivel-type 
core barrels, one 5 ft and one 7 ft in 
length, and an EWX single-tube core bar- 
rel 2 ft in length (fig. 2). 

3. EX (1-1/2-in diam) diamond bits. 

4 X denotes manufacturer's series of 
equipment. 




FIGURE 1. - Type CP-65 air dri 




FIGURE 2. - EX-size bit with core spring, reamer, and 2-ft EWX core barrel 



4. A reamer for use with the EX bit 
in the EX pilot gauge hole. 

5. EW drill rod in the following 
lengths: six 5-ft pieces, four 2-ft 
pieces, and two 1-ft pieces. 

6. Two stabilizers (5-7/8 in. in diam 
by 8 in long) on a 2-ft length of EW 
drill rod (fig. 3). (Note. — The stabi- 
lizer can be made from a used 6-in-diam 
core barrel cut 8 in long.) 

7. BX wire line drill rod (fig. 3) in 
the following lengths: three 5-ft pieces, 



four 2-ft pieces, and two 1-ft pieces. 
BX wire line drill rod has been found 
best for overcore drilling. The inside 
diameter of this drill rod is large 
enough for the borehole gauge and place- 
ment tool to pass through. Also, the 
wire line drill rod is lightweight yet 
has the necessary strength for overcore 
drilling requirements. The rod has four 
threads per inch, which is desirable to 
provide fast coupling. 

8. Two stabilizers (5-7/8 in. in 
diam by 8 in long) on a 2-ft length of BX 
wire line drill rod. 




FIGURE 3. - Stabilizer on 2-ft section of EW drill rod (upper left), stabilizer on 1-ft section of BX 
wire line drill rod (upper right), and 2-ft piece of BX wire line drill rod (foreground). 

9. An adapter sub (EW box to BX wire 
line pin) for connecting the EW drill rod 
in the chuck, and quill to the BX wire 
line drill rod. 

10. A water swivel having a plug with 
a 1/2-in hole to allow threading of the 
gauge cable when it is in use and an add- 
itional solid plug to fit when the cable 
is not in use (fig. 4). The water swivel 
connects to the EW drill rod in the 
drill. (Note. — The EW pins at the water 
swivel and at the chuck must be drilled 
out to at least 9/16 in ID to allow cable 
and drilling water to pass through.) 

11. An expander head for connecting 
the BX wire line drill rod to the 6-in-OD 
diamond overcoring bit (figs. 5-6). 

12. Thin-wall masonry overcoring bits 
6 in. in diam and long enough to obtain 
18 in of overcore 5-5/8 in. in diam (fig. 
6). 

13. A 6-in-diam starter barrel 1 ft 
in length, equipped with a detachable 1- 
1/2-in-diam pilot shaft in the center 
that extends approximately 5 in beyond 
the diamonds of the starter barrel. This 
barrel is used to center the 6-in-diam 
hole over the initial EX hole (figs. 7- 
8). 




FIGURE 4. - Water swivel with solid plug and plug 
(right) used during overcoring. 

14. An EW core barrel to replace the 
detachable pilot shaft, sized to extend 1 
in beyond the starter barrel. With the 
bit and reamer attached, this unit is 
used to drill an EX starter hole 4 in 
deep in the bottom of the 6-in-diam hori- 
zontal or vertical hole. 

15. A 6-in-diam centering stabilizer 
for starting the 5- or 7-ft EWX core bar- 
rel into the 4-in-deep horizontal starter 
hole or for centering the EWX core barrel 
in the bottom of a 6-in-diam vertical 
hole (fig. 9). 




FIGURE 5. - Core barrel, 6 in. in diam by 27 in long (referred to as a 2-ft core barrel), and 
disassembled expander head. 




FIGURE 6. - Expander head assembled (adapted for BX wire line drill rod) and 6-in-diam core barrel 



t . 

<- EX (IVin diam) hole 
Drill7ftdeep. 




^Expander head 



Chuck 



iV 



^-EW rod 



0_ 



FIGURE 7. - Starting the 6-in-diam overcoring hole. 




FIGURE 8. - Starter barrel, 6 in. in diam by 1ft long, 
with pilot and expander head adapted for EW drill rod. 



FIGURE 9. - Centering stabilizer. 



Diameter of mine opening 



EX bit-^ \J\ rDiamonds 



^L 



*-„ — "5* 

Reamer-^ ^ 



^Core barrel 



Steel ring-' vr on t»rinn c+nhiliTor 



EW rod -' 



Chuck. 



~J 



Centering stabilizer 



.-Face 



NOTE.-- ID of steel ring must be less 
than OD of diamonds of reamer. 



FIGURE 10. - Starting the EX hole in the bottom of the 6-in-diam hole. 



16. A steel retriever ring 2 in. in 
OD, 1.448 in. in ID, and 3/4 in wide 
placed on the EWX core barrel in front of 
the centering stabilizer for retrieving 
the centering stabilizer (fig. 10). 

17. A core breaker to fit the EW rod. 
The core breaker, constructed of steel, 
should be at least 2-1/2 in wide and 
hardened (fig. 11). 

18. A 6-in core shovel to fit an EW 
rod for retrieving the core from a hori- 
zontal hole (fig. 11). 

19. A 6-in core puller approximately 
18 in long to fit an EW drill rod for re- 
trieving the core from a vertical hole 
(fig. 11). The attachment end of the 
barrel should have an approximately 5/8- 
in-thick steel plate welded around its 
circumference to the barrel, with an EW 
box welded in the center of the plate. 



Three 1-1/2-in-diam holes on 120° centers 
are drilled through the plate to allow 
water to pass when the core puller is 
lowered into the 6-in-diam hole. The 
downhole end of the barrel has four U- 
cuts 90° apart. The remaining rectang- 
ular sections of metal are bent in toward 
the center slightly to hold the 6-in core 
in the barrel when lifting it out of the 
vertical hole. The core puller must be 
marked and kept from rotating as it is 
lowered into the 6-in-diam hole, to en- 
sure orientation of the core. (Note. — 
The shovel and core puller should be 
made from a used 6-in-diam core barrel.) 

An alternate means of retrieving an in- 
tact 6-in core having an EX-size hole in 
it from a vertical hole is by means of a 
roof bolt anchor. A diamond shaped steel 
point must be welded on the front of the 
anchor (fig. 12). A section of threaded 




FIGURE 11. - Core breaker (lower right), core shovel (lower left), and core puller (center) 



■To 


i i i i 


- . 

5 6 

I 1 




■■' ■ *J 


■■' 




Scale, in 









FIGURE 12. - Retrieval tool used in place of core puller to retrieve 6-in-diam core from a vertical 
hole that has an EX hole through the core. 

rod is then screwed into the anchor, and are used to extend this tool into the 
sections of 3/8-in pipe, 5 ft in length, hole. 

INSTRUMENTATION 



The following instruments are required 
for calibrating, testing the drilled 
core, and measuring the deformations dur- 
ing the overcoring: 

1. Three-component (3-D) borehole 
gauge and accessories (including special 
pliers, 0.005- and 0.015-in-thick. brass 
washers, and silicone grease) (fig. 13). 
A report on the assembly and wiring of 
the gauge is available (2) . This inform- 
ation is useful when minor repairs on the 
gauge are required. 

2. Three Vishay model P-350 5 or 
equivalent strain indicators (fig. 14). 

-"Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



(Note. — Normally, a gauge factor (G.F.) 
of 0.40 is used so that an indicator unit 
(1 yin/in) represents approximately 1 uin 
displacement. If other indicators are 
used and a G.F. of 0.40 cannot be obtain- 
ed, then calibration at a G.F. suitable 
to the indicator used is necessary.) If 
only one indicator is available, a 
switching and balancing unit or a switch- 
ing unit must be used. 

3. Orientation and placement tool for 
the gauge (fig. 15). 



4. A calibration device for 
(fig. 14). 



the gauge 



5. A biaxial chamber for determining 
the modulus of elasticity of the rock 
(fig. 16). 




O I 2 3 4 5 6 

iill I 1 I 

Scale, in 





I t o i 



p 



FIGURE 13. - Special pliers, Bureau of Mines three-component borehole gauge, piston, disassembled 
piston and washer, and transducer with nut. 




Scale, in 



FIGURE 14. - Strain indicator with calibration device (left) and switching unit (right). 




FIGURE 15. - Placement and retrieval tool. 



SITE SELECTION 



To calculate the complete stress ellip- 
soid, deformation measurements must be 
obtained in at least three nonparallel 
holes. Typical layouts of boreholes and 
directions of measurement are shown in 
figure 17. Three orthogonal boreholes 
provide the best configuration of three 
boreholes for determining all six stress 
components with uniform precision 
(fig. 17 A). It is possible to obtain 
very good results from configurations 
175, C, and D, where the angle between 
the horizontal holes has been decreased 
from 90° to not less than 60°. Deform- 
ation measurements should be made outside 
the zone of influence of the mined 



opening. This distance is usually taken 
to be one diameter of the mine opening. 
Borehole configurations 17^4, B, and C re- 
quire only one drill setup. Geologic or 
structural conditions may require that 
the zone of stress measurement be confin- 
ed to a small area. The configuration 
17D will provide good engineering esti- 
mates of the stress components, but a 
high standard deviation will exist for 
any calculated principal stress component 
with an orientation parallel or subparal- 
lel to the axis of any of the boreholes. 
This configuration also requires three 
drill setups. 



10 








12 3 4 

III! 








L_ 


5 6 1 
1 1 I 


Scale, in 



FIGURE T6. - Biaxial chamber and pump. 




.e,o°-?o° 



^j/^j&M^s/hakMM. 



D 



°Vertical downhole 





FIGURE 17. - Drill hole configurations 
diameter of mine opening.) 



(D ind icates 



Horizontal holes should be started 3° 
upward from horizontal to facilitate re- 
moval of water and drill cuttings from 
the hole. For holes longer than 25 ft, a 
5° up-angle is necessary. The drill site 
should be located in competent rock where 
core lengths of at least 1 ft can be 
obtained. Sometimes it may be advisable 
to drill NX holes at the proposed over- 
coring location prior to beginning the 
actual overcoring operation to determine 
if cores of sufficient length can be 
obtained. 

Recent improvements (5) to the borehole 
gauge now allow for a complete stress re- 
lief in cores of 6-in length. At the 
present time, the minimum length of core 
required to perform a biaxial test in the 
field for the modulus of elasticity is 6 
in. 



OVERCORING PROCEDURE 



11 



For determining stress distribution 
near the mine opening, overcoring is 
started at the face and the following 
procedures apply: 

1. Drill 7 ft of EX pilot hole. This 
distance is the usual limit of pilot-hole 
depth because overcoring in longer holes 
off-centers the pilot hole to such a de- 
gree that the cores cannot be tested for 
Young's modulus. However, the deform- 
ation readings will not be affected if 
the pilot hole becomes nonconcentric with 
the 6-in-diam hole. For the test to de- 
termine Young's modulus, the pilot hole 
must be at least 1-1/4 in from the cir- 
cumference of the 5-5/8-in-diam core cut 
by the 6-in-diam thin-wall diamond bit. 

2. Use the 6-in-diam starter barrel 
to drill approximately 1/4 in of kerf so 
that the overcoring barrel can be started 
(fig. 7). 

3. Uncouple the starter barrel from 
the drill rod near the chuck and remove 
the starter barrel. 

4. Open the drill to provide clear- 
ance for the borehole gauge placement and 
retrieval tool. 

5. String the taped end of the gauge 
cable through the adapter sub, EW drill 
rod, and water swivel (fig. 18). 



6. Remove the tape from the gauge 
cable and connect the cable to the three 
strain indicators as shown in figure 19. 
Then remove the pistons with the special 
pliers, grease with silicone grease, and 
replace in their proper place in the 
gauge. (If a piston is accidentally 
dropped, foreign materials can be removed 
from the O-ring with a toothbrush and the 
piston wiped clean with a cloth. The 0- 
ring is then greased and the piston re- 
placed into the gauge.) To assure that 
the transducers across the diameters are 
properly connected to the strain indica- 
tors, apply a slight pressure with the 
thumb on each piston in turn while watch- 
ing the indicator. The respective indi- 
cator needles will move as the pressure 
is applied to the piston. 



Examine the case screws of 
and tighten them if necessary. 



the gauge 



Then pull the six pistons about halfway 
out of the gauge using the special pli- 
ers, to ensure they are not touching the 
transducers. Take a zero reading for 
each diametral component and record them 
on the field data sheet (fig. 20) in the 
row labeled "Zero," in the three columns 
labeled "U,," "U 2 ," and "U 3 ." Push the 
pistons back to their original position. 

If only one indicator is available, 
a switching and balancing unit or a 



m i. 



6-in-diam diamond bit 27 in long 

r BX wire line drill rod 




"3-D borehole gouge 



,EW rod 



o 



-Column (for mounting drill) 
^Drill (open position) 



Cable going to 
strain indicator, 




"-Qui 1 1 

Water swive 
Water hose- 




FIGURE T8. - Setup for three-component (3-D) borehole gauge emplacement. 



12 



Sensitivity knob: turn full clockwise. 

Balance knob: put in midrange (5 turns of the IO-turn potentiometer). 

Bridge switch: to full. 



GALV SENSI 

° O 

SCOPE 

o obat. test 

BALANCE 




Brown 
8-conductor cable No. 18 



NOTE. — Hook black and green wires to indicator 2 and use 2 other wires (No. 18 or No. 20) 
to common P+ and P-(or ^ and P„ ) of all 3 indicators. 

FIGURE 19. - Wire hookup to strain indicators. 



switching unit must be used. If no 
switching unit is available, proceed as 
follows: (1) Hook up the indicator as 
shown in figure 19, first indicator, (2) 
take a reading U], (3) replace the white 
and red wires with the yellow and orange 
wires, (4) take another reading TJ 2 , (5) 
replace the yellow and orange wires with 
the brown and blue wires, and (6) read 
U 3 . 

It is strongly recommended that three 
strain indicators be used when overcor- 
ing. This allows the indicator reader to 
monitor all three components of the gauge 
simultaneously, and if a fracture or min- 
eralized vein separates, it can be de- 
tected and noted on the field data sheet 
(fig. 20) so that the deformation reading 
can be corrected. Also, a breaking core 
will be immediately detected and the 
drill operator notified so that the drill 
can be stopped before the gauge is damag- 
ed. When a switching unit is used and 



the core happens to break while the unit 
is being switched from one channel to an- 
other, the gauge will spin with the 6-in- 
diam barrel. This will twist the cable 
and tear the wires from the connecting 
plug, resulting in a major gauge repair. 
Also, if a fracture or mineralized vein 
opens but does not completely separate 
during channel switching, it will not be 
detected. Thus, without anyone's know- 
ing, the deformation reading(s) would be 
erroneous. 

7. Place the 6-in-diam core barrel in- 
to the kerf cut by the starter barrel. 

8. Holding the gauge with the notch 
indicating Uj (component 1) up, engage 
the orientation pins of the gauge with 
the placement and retrieval tool using a 
clockwise rotation. The notch is filed 
in front of the piston hole in the gauge 
case. 



13 



Hole No. 
Gouge No. 



Dote. 



Gauge factor 



True bearing of hole. 



Orientation ! U| 

Calibration factor U| 

U 2 

u, 



DEPTH 




DEFORMATION 


TIME 


TEMP. 


REMARKS 


Gauge 


Hole 

( + ) 


INDICATOR READING 


Gauge 
Set 


Overcore 
Start 


Deformation 
Read 


Rock 


Water 


U| 


U 2 


u 3 








Zer o 




















9" 


Face 


Bias 






















1/2 
























l" 
























1 1/2 
























2" 
























oj^jj 























7^2 



8^2 



Plstoni 



9" 



9/2 



10" 



10/2 





13 






















l3j/ 2 " 






















14" 






















14^ 






















15" 






















1 5 Yi 






















16" 






















l6'/2 






















17 






















17^' 






















16 





















NOTE.-- Next relief would start at 18 in and go to 36in; gauge 
would be orientated at a depth of 27 in. 



FIGURE 20. - Field data sheet. 



14 



POSITION OF COMPONENTS 

u, 

U 2 ^--v-\ U 3 Notch in gauge (at U |) 




Viewed from cable end of gauge. 



The clockwise rotation sets and orients 
the gauge. Since U] (the notch in the 
gauge) is perpendicular to the orienta- 
tion handle, a small level can be used on 
the handle for orientation in horizontal 
holes. (Note. — If the gauge is oriented 
too far clockwise, a counterclockwise 
motion of more than 120° will allow the 
gauge to be moved counterclockwise, then 
reoriented correctly with the clockwise 
rotation. ) 

9. With the placement rod, put the 
gauge through the 6-in-diam core barrel 
and into the EX hole 9 in past the 6- 
in-diam kerf, and orient the gauge (fig. 
21). (Note. — The 9-in gauge placement 
can be reduced to 6 in, thereby reducing 
the total length of the overcore from 18 
to 12 in, and good results will still be 
obtainable. A further reduction in core 
length can be obtained using the reversed 
gauge case (_5_) in rock that consistently 
breaks prematurely.) 



10. Check the bias of the gauge on 
the strain indicators. (See the next 
section, "Procedure for Operating the 
Three-Component Borehole Gauge," for 
amount of bias needed and for changing 
bias. ) 

11. Turn the placement tool counter- 
clockwise (approximately 60°) to free it 
from the gauge. 

12. Remove the tool from the hole. 

13. Close the drill. 

14. Pull all excess cable through the 
drill. 

15. Couple the EW drill rod in the 
chuck to the BX wire line drill rod ex- 
tending out of the drill hole. 

16. Hold or tie the cable behind the 
water swivel. 



, EX hole 



f Borehole gauge cable 

BX wire line drill rod -\ 
i i 



( Placement and retrieval 
* tool 

/Level 




Wood taped to placement 
rod to hold placement 
rod in center. 



^-Face 



Orientation 
handle 



FIGURE 21. - Positioning three-component (3-D) borehole gauge with placement rod through the 6-in- 
diam core barrel and drill rod. 



15 



17. Turn on water. Allow approxi- 
mately 10 min from the time the gauge is 
set for the gauge, drill water, and rock 
to come to temperature equilibrium. 

18. Start the overcore with a chuck 
speed of approximately 120 rpm and a pen- 
etration rate of 1/2 in per 40 s; 120 rpm 
gives very satisfactory bit life and will 
lessen the chance of damaging the gauge 
if the core should break during over- 
coring. (Note. — If the core breaks dur- 
ing overcoring, this breakage will be 
shown by the indicators fluctuating or 
the cable twisting through the drill rod. 
If this happens, the drill should be shut 
down immediately and the gauge and broken 
core retrieved.) 

Each 1/2-in penetration should be sig- 
naled to the indicator reader, and the 
indicator readings (deformation measure- 
ments) should be recorded on the field 
data sheet (fig. 20). The rate of pene- 
tration of 1/2 in every 40 s allows just 
enough time for recording three diametral 
readings. Overcoring should proceed for 
a total of 18 in, which is 9 in beyond 
the plane of measurement. This will re- 
sult in 36 sets of readings. (See note 
in step 9.) 

19. Upon completion of overcoring or 
if the core breaks prematurely, uncouple 
the BX wire line drill rod at the adapter 
sub. 

20. Pull a little slack in the cable 
so the drill can be opened. 

21. Hook the retrieval tool onto the 
gauge cable with the wire clip and insert 
it through the BX wire line drill rod 
while holding the handle of the tool 
counterclockwise approximately 60° from 
horizontal. Then push the retrieval tool 
into the borehole until the tool slips 
onto the gauge. Rotate the handle clock- 
wise until the tool stops against the 
orientation pins on the gauge. Remove 
the gauge from the borehole. 

22. Remove the core barrel and drill 
rod from the hole. 



23. Use the core breaker to break 
core. 

24. Retrieve the core with the core 
shovel in a horizontal hole or with the 
core puller in a vertical hole. Exercise 
great care not to rotate the core puller 
as it is lowered into the 6-in-diam hole, 
to ensure orientation of the core. Mark 
the orientation, depth, and plane of mea- 
surement on the core. 

25. Place the 6-in-diam core barrel 
and drill rod back into the hole. 

26. Repeat steps 8 through 25 for 
each additional set of readings in the 
remaining EX pilot hole. Do not position 
the gauge pistons closer than 12 in to 
the bottom of the pilot hole. 

Overcores should be tested in a biaxial 
chamber as soon as conveniently possible 
after recovery to determine the modulus 
of elasticity. 

The 7-ft EX hole allows for approxi- 
mately five complete overcores. After 
they are completed, the remaining EX hole 
is drilled out, a new pilot hole is 
drilled, and additional overcores are 
performed. Stabilizers are inserted in 
the drill stem at 10-ft intervals or when 
drill rod vibration occurs. 

In all drilling, the EX pilot hole 
should be cored with a double-tube barrel 
so that the core can be obtained for ob- 
servations. This core indicates the 
fractures and/or if core disking is oc- 
curring so that the gauge can be placed 
in the competent zones. Disking areas or 
highly fractured areas should be drilled 
out. 

If the only interest is to determine 
the stress field outside the influence of 
the mine opening, time can be saved by 
first drilling a 6-in-diam hole without 
instrumentation to a depth of one diame- 
ter of the mine opening. Then, centered 
in the bottom of the 6-in-diam hole, 
approximately 7 ft of EX hole is drilled 
for gauge emplacement and subsequent 



16 



overcore determinations (fig. 10) . At a 
distance of one diameter from the mine 
opening, at least three good sets of 
readings are required from each hole in 
order to obtain a good statistical de- 
termination of the average ground stress 
components. 



Surface topography influences the grav- 
itational stress in underground openings. 
The loading values attributable to to- 
pography should be calculated (7) and 
compared with the calculated stresses de- 
termined from the overcoring. 



PROCEDURE FOR OPERATING THE THREE-COMPONENT BOREHOLE GAUGE 



The procedures described in this sec- 
tion have been developed through use of 
the borehole gauge over a long period of 
time. It is important that the details 
be followed closely in order to minimize 
any chance of damage to the gauge and 
to provide the most reliable data 
acquisition. 

1. Hook the gauge to the indicators, 
as shown in figure 19, or to a switching 
unit and one indicator. 

2. Remove all the pistons from the 
gauge with special pliers (fig. 13). The 
special pliers are made by brazing a 1/2- 
in-diam by 3/8-in-long brass rod to the 
jaws of a standard pair of pliers. The 
cylindrical axis of the rod is parallel 
to the handles of the pliers. ^ An 11/32- 
in-diam hole is drilled axially through 
the center of the brass. The pliers are 
opened by sawing through the brass along 
the diameter perpendicular to the handles 
of the pliers leaving a half-round piece 
of brass attached to each jaw. The out- 
side edges are then filed to fit inside 
the 1/2-in-wide milled flats of the gauge 
case. 

3. Record the zero reading for each 
of the three diametral components. Label 
them U lf U 2 , and U 3 zero reading. 

4. Grease the 0-ring on each piston 
and insert the pistons into their proper 
place in the gauge. 

5. Place the gauge into the EX hole 
or into a test specimen if the work is 
being done in the laboratory. Caution. — 
Do not force the gauge into the EX hole. 

"See the appendix, figure A-1 . 



If it fits very tightly, remove washers 
from the pistons as follows: 

a. Remove one piston of a diame- 
tral pair with the special pliers. 

b. Hold the piston with both pli- 
ers and screw it apart, being careful 
not to grasp the 0-ring with the 
pliers. 

c. Remove a washer and screw the 
piston firmly back together. 

d. Grease the 0-ring and insert 
the piston into the gauge. 

e. Repeat steps a through d for 
the remaining two diametral pairs. Re- 
set the gauge in the EX hole. If the 
gauge is still too tight, remove a 
washer from the remaining three pistons 
as stated in steps a through d. If up- 
on the initial insertion the gauge is 
very loose inside the EX hole, remove 
the pistons one at a time and add a 
washer using steps a through d. 

6. When the gauge is properly sized, 
it should offer minimal to moderate re- 
sistance as it is placed into the EX 
hole. After the gauge is inserted to the 
proper depth, position it in the desired 
orientation. 

7. Read all three components (U 1 , U 2 , 
and U 3 ). 

When relieving stress, as in field 
overcoring, the bias set on each compo- 
nent should be between 13,000 and 20,000 
indicator units (microinches per inch) , 
with a G.F. of 0.40. When stress is 
applied to a specimen, as in the 



17 



laboratory or in the biaxial chamber, the 
bias set on each of the components should 
be between 8,000 and 15,000 indicator 
units for the same G.F. 

The following tabulation shows typical 
zero readings and calibration factors for 
a G.F. of 0.40: 

Zero readings (pistons out), uin/in: 

U, = -4,000 

U 2 « +3,100 

U 3 a +4,100 
Calibration factors, uin per pin/in: 

U, = 1.00 

U 2 = 1.00 

U 3 = 1.04 



Care must be exercised not to overload 
the transducers, since not all borehole 
gauges have the element safety plug in- 
stalled in the gauge case. Maximum load 
on any component must not exceed 50,000 
indicator units with a G.F. of 0.40. 
The purpose of the element safety plug is 
to restrict the end displacement of the 
transducer to less than 0.025 in. 

Lowering the G.F. increases sensitiv- 
ity. A G.F. of 0.40 provides a reading 
of one indicator unit for 1 uin of 
deformation. 

When a G.F. other than 0.40 is used, 

the working range and calibration factor 

must be determined in advance of 
operating. 



PROCEDURE FOR BIAXIAL TESTING OF THE OVERCORE TO DETERMINE 
ITS ANISOTROPIC PARAMETERS 



The cores recovered from the overcoring 
stress relief should be biaxially tested 
(4^) as soon as possible to determine the 
modulus of elasticity (E). Generally, 
the cores recovered from an overcoring 
hole are tested while the setup is made 
on the second hole and preliminary drill- 
ing performed. The core to be tested 
must be intact for at least 6 in. Cores 
cut with a used bit will fit the testing 
apparatus properly. Those cut with a new 
bit may require one wrap of 0.015- 
in-thick plastic to fit properly. The 
core, either wrapped or unwrapped, is in- 
serted into the biaxial chamber with the 
plane of measurement centered in the 
chamber and with the orientation mark 
(position U ( ) at the 12-o'clock position. 
If it is not possible to center the plane 
of measurement, it should be positioned 
in the center third of the chamber keep- 
ing U] in the 12-o'clock position. The 
clearance is checked between the circum- 
ference of the core and the biaxial 
chamber at the 12-o'clock position. If 
there is more than approximately 1/16 in 
(0.06 in) clearance, then the core must 
be centered in the chamber with wooden 
wedges to prevent the liner from "popping 
out" and releasing oil when pressure is 
applied. This would also require repair- 
ing the liner and chamber. The core is 



centered by using four pieces of wood 
approximately 1/32 in thick, 1/8 in wide, 
and 3 to 4 in long with the last inch 
beveled to an edge. Place two wedges at 
each end between the core and the chamber 
at the 4:30 and 7:30 positions, inserting 
the wood about 3/4 in. Then apply about 
100 psi to the core while the valve is 
slowly opened to remove any entrapped air 
in the system. Repressure the core 
(about 50 psi) to hold it rigid, then em- 
place the borehole gauge. 

The borehole gauge is positioned and 
oriented at the same position in the core 
as it was during the overcoring stress 
relief, if possible. The core is pre- 
loaded three times to a selected stress 
level and then a set of data is recorded. 
(Note. — Care must be taken not to select 
a stress level that exceeds the tensile 
strength of the rock in the axial direc- 
tion: otherwise, the core will fail in 
tension and the test for the modulus of 
elasticity will be lost. Values for the 
stress level should range from 1,500 psi 
for granites down to 250 psi for tuff or 
fractured rock.) A run consists of (1) 
recording a no-load reading for each com- 
ponent (Uj, U 2 , and U 3 ), (2) pressuring 
the core to the selected stress level and 
reading each component, and (3) releasing 



the pressure and recording 
readings after unloading. 



the component 



The gauge is rotated counterclockwise 
within the core 15°, preloaded twice, and 
another run made. These three sets of 
readings are labeled U]+15, U 2 +15, and 
U3+I5. This procedure is followed three 
more times. The last run is a repetition 
of the first run, except U 3 is now posi- 
tioned at Uj, U| at U 2 , and U 2 at U 3 . 
The three sets of readings from the third 



run are labeled U,+30, U 2 +30, and U 3 +30. 
The fourth run is labeled U,+45, U 2 +45 , 
and U3+45. The fifth or redundant read- 
ings are U,+60, U 2 +60, and U3+6O. 

Root reciprocals [U]"' /2 of these 12 
readings are plotted, and a least squares 
fit (8^) is performed to determine the 
maximum and minimum axes of the defor- 
mations. From these data, the maximum 
and minimum E values or anisotropic par- 
ameters of the core are obtained. 



BOREHOLE GAUGE CALIBRATION 



The following describes the procedure 
for calibrating the three-component bore- 
hole gauge. The gauge should be cali- 
brated before it is used. It is a good 
practice to recalibrate the gauge after 
use, also, to assure that no damage has 
occurred to it. 

1. Grease all pistons. 

2. Put them into the gauge. 

3. Place the gauge into the calibra- 
tion jig. 

4. Position the gauge so that the 
pistons for component 1 (U j) are visible 
through the micrometer holes. 

5. Tighten the three wingnuts. 

6. Install the two micrometer heads 
and lightly tighten the set screws. 

7. Set the strain indicator on "Full 
Bridge," center the balance knob if there 
is one, and set the G.F. at 0.40 or the 
factor desired. (Note. — The G.F. set- 
ting for calibration must be the same as 
that used in the field for overcoring and 
biaxial testing.) 

8. Hook the wires for component 1 
(Ui) (black, green, white, and red) to 
the indicator as shown in figure 19 and 
balance it. 

9. Turn one micrometer in, until the 
needle of the indicator just starts to 



move. The micrometer is now in contact 
with the pistons. 

10. Do the same with the opposite 
micrometer. 



11. Rebalance 
necessary. 



the indicator if 



12. Record this no-load reading (zero 
displacement). 

13. Turn each micrometer in 0.0160 in 
(a total of 0.0320 in displacement for 
component 1). (Note. — The micrometer 
reads to ten-thousandths of an inch. ) 

14. Balance the indicator. 

15. Record the reading. 

16. Wait 2 min and check the combined 
creep for the two transducers. Creep for 
2 min should not exceed 20 yin/in. 

17. Record the new reading. 

18. Back off each micrometer 0.0040 
in (a total of 0.0080 in for the 
component) . 

19. Balance and record. 

20. Continue this procedure until the 
micrometers are back to the zero posi- 
tion. This reading is also the zero dis- 
placement reading for the second run. 



19 



21. Repeat steps 13 through 20 for 
the second cycle. 

22. Loosen the wingnuts and rotate 
the gauge clockwise to line up component 
2 (U 2 ) with the micrometers. 

23. Retighten the wingnuts. 



24. Hook up the wires for component 2 
(U 2 ) (black, green, yellow, and orange) 
to the strain indicator. 

25. Repeat steps 9 through 21 for com- 
ponent 2. 

26. Calibrate component 3 (U 3 ) in a 
similar manner. 



CALIBRATION DATA ANALYSIS 



The following is an example using two runs for one 
of 0.40: 



component, calibrated at a G.F. 



Displacement, in 



Indicator reading, uin/in Difference, pin/in 






RUN 1 






-693 


NAp 


.0320 


+30,140 


( 2 ) 


.0320 


30,055 


30,535 


.0240 


21,920 


22,400 


.0160 


14,040 


14,520 


.0080 


6,380 


6,860 





-480 


NAp 



RUN 2 






-480 


NAp 


.0320 


+30,034 


( 2 ) 


.0320 


29,980 


30,430 


.0240 


21,914 


22,364 


.0160 


13,975 


14,425 


.0080 


6,335 


6,785 





-450 


NAp 



NAp Not applicable. 

i„ , ., . ,. Known displacement (uin) 

Calibration factor: — 

2 Wait 2 min. 



Indicator units 



uin/indicator unit. 



Subtract the zero displacement reading 
(last reading of each run) from all the 
other indicator readings to determine the 
differences. These values are in micro- 
inches per inch. Because of friction be- 
tween the piston 0-ring and the wall of 
the hole in the gauge case, the least 
difference value (in run 1, 6,860 uin/in 
at 0.0080-in displacement) is subtracted 
from the largest difference value (30,535 
uin/in at 0.0320 in). This difference 
(23,675), in microinches per inch, is 
divided into the deformation difference 
for the given range; in this case 0.0240 
in or 24,000 yin. The result is the cal- 
ibration factor. Repeat these calcula- 
tions for the second run and average them 



to obtain 
ponent 1. 



the calibration factor for corn- 



Thus , for run 1 , 



and for run 2, 



24,000 
23,675 



= 1.014, 



24,000 . 
23,645 

Use a calibration factor 
the component. 

Repeat the above procedure 
the calibration factors for 
and 3. 



1.015. 



of 1.01 for 



to determine 
components 2 



20 



The above method is a faster means of 
calculation and yields results accurate 
to within 1 pet of the least squares 
method. However, if desired, a least 



squares determination can be made using a 
plot of micrometer displacement versus 
indicator units. 



IDENTIFICATION OF SOURCE OF BOREHOLE GAUGE MALFUNCTION 



LACK OF BALANCE ON ONE 
OR MORE INDICATORS 

If balance on one or more of the indi- 
cators is not achieved, recheck the wir- 
ing hookup to see that it is in accord- 
ance with the wiring diagrams in figure 
19 and that all connections are tight. 
If balancing is still not achieved, the 
problem may be in the plug-in cable con- 
nection to the borehole gauge. This is 
checked by removing all of the screws 
from the placement end of the gauge, re- 
moving the end, turning off the knurled 
clamping nut, and pushing in the cable to 
ensure good connection. With the indi- 
cators set to their respective zero read- 
ings, each component will provide indi- 
cator balance when a good connection is 
made. Screw the knurled clamping nut 
very tightly into place and replace the 
end. (Note. — Nonbalance may occur when 
too much pull has been exerted on the 
cable accidentally or intentionally dur- 
ing gauge retrieval after overcoring. 
Sometimes in a vertical hole, cuttings or 
broken rock drop into the EX hole, imped- 
ing the gauge retrieval tool from hooking 
onto the orientation pins of the gauge. 
Do not attempt to retrieve the gauge by 
pulling the cable. Instead, remove the 
drilling rod and the 6-in-diam core bar- 
rel and break off the core with the gauge 
still in place. Remove the gauge cable 
from the drill, the 6-in-diam core bar- 
rel, and the drill rod. String the cable 
through one of the holes in the end of 
the core puller and then retrieve both 
core and gauge with the core puller. ) 

INSENSITIVITY OF ONE OR MORE ELEMENTS 
ON INDICATOR 

If elements become insensitive to de- 
flection of the pistons or nonresponsive 



to the turning of the indicator dial, the 
probability exists that moisture has en- 
tered the connecting plug or cable. In 
this case, do the following: 

1. Remove the pistons and the bore- 
hole gauge case to check for water. If 
water is present, check the piston 0- 
rings for a possible cut that may have 
occurred from gripping the 0-ring with 
pliers. Grease the 0-rings and replace 
the case. 

2. Remove the placement end as de- 
scribed previously. Check the grommet 
seal. If water is found inside the gauge 
or in the cable connecting plug, dry out 
completely, regrease the cable where it 
passes through the grommet, and tighten 
the knurled nut firmly. Replace the 
placement end. 

LACK OF COMPONENT BALANCE 

If one component does not balance any- 
where on the indicator dials or balances 
intermittently, this situation indicates 
a disconnected wire or possibly a cold 
solder joint. Remove the borehole gauge 
case and check all wires going to this 
component including the plug-in cable 
connector. Solder where needed and re- 
place the gauge case. 

SENSITIVITY OF INDICATORS WHEN TOUCHED 

If indicators are sensitive when being 
touched, this sensitivity generally oc- 
curs after the instruments are used for a 
period of time in very humid conditions. 
Use plastic or other insulating material 
underneath the indicators during the 
working day. Each night the indicators 
should be brought out of the mine and 
allowed to dry. 



REFERENCES 



21 



1. Becker, R. M. , and V. E. Hooker. 
Some Anisotropic Considerations in Rock 
Stress Determinations. BuMines RI 6965, 
1967, 23 pp. 

2. Bickel, D. L. Transducer Prepara- 
tion and Gage Assembly of the Bureau of 
Mines Three-Component Borehole Deforma- 
tion Gage. BuMines IC 8764, 1978, 19 pp. 

3. Duvall, W. I., and J. R. Aggson. 
Least Squares Calculation of Horizontal 
Stresses From More Than Three Diametral 
Deformations in Vertical Boreholes. Bu- 
Mines RI 8414, 1980, 12 pp. 

4. Fitzpatrick, J. Biaxial Device 
for Determining the Modulus of Elasticity 
of Stress-Relief Cores. BuMines RI 6128, 
1962, 13 pp. 

5. Hooker, V. E., J. R. Aggson, and 
D. L. Bickel. Improvements in the Three- 
Component Borehole Deformation Gage and 
Overcoring Techniques. BuMines RI 7894, 
1974, 29 pp. 

6. Hooker, V. E., and D. L. Bickel. 
Overcoring Equipment and Techniques Used 



in Rock Stress Determination. 
8618, 1974, 32 pp. 



BuMines IC 



7. Hooker, V. E., D. L. Bickel, and 
J. R. Aggson. In Situ Determination of 
Stresses in Mountainous Topography. Bu- 
Mines RI 7654, 1972, 19 pp. 

8. Hooker, V. E., and C. F. Johnson. 
Near-Surface Horizontal Stresses Includ- 
ing the Effects of Rock Anisotropy. Bu- 
Mines RI 7224, 1969, 29 pp. 

9. Merrill, R. H. Three-Component 
Borehole Deformation Gage for Determining 
the Stress in Rock. BuMines RI 7015, 
1967, 38 pp. 

10. Obert, L. Triaxial Method for 
Determining the Elastic Constants of 
Stress Relief Cores. BuMines RI 6490, 
1964, 22 pp. 

11. Panek, L. A. Calculation of the 
Average Ground Stress Components From 
Measurements of the Diametral Deformation 
of a Drill Hole. BuMines RI 6732, 1966, 
41 pp. 



22 



APPENDIX.— EQUIPMENT DIAGRAMS 

Working drawings of this equipment are Bureau of Mines, Building 20, Denver Fed- 
available from Denver Research Center, eral Center, Denver, CO 80225. 



fj 



11 /16" 

Element safety plug 

No 303 stainless 
1 required 




»J 



Gauge case 

No 303 stainless 
1 required 



Section A-A 




216" 



336" 109" 

Wear 

button Piston assembly 

Kennametal 6 No. 303 stainless 
6 required 6 required 



T 

i 

9/32" 

A 



Piston 
washer 

Brass 




Reverse case 

No 303 stainless 
1 required 



End view 



U 2" »J 

Locking nut wrench 
Tool steel 



Connector emplacement tool 

Brass 
1 required 



13/32" 

Pin 

Brass 

9 required 




'Special' pliers 





13/32" 

Orientation pin 

No 303 stainless 
2 required 



1-1/8 

Section A-A 



-Jul 



~v 



-5-1/2" (7-1/2") - 
«J 

Placement end 

No 303 stainless 
1 required 




Section B-B 



U J 

3/4" 



Clamping 
nut 

No 303 stainless 
1 required 



Grommet 5/8 " 
washer _ 
Aluminum Grommet 

Rubber 
1 required 



1 required 





3-3/8" - 



Gauge body 

No 303 stainless 
1 required 




End view 



1 /4" 

Locking nut 

No 303 stainless 
6 required 



U 2-7/16" »-t 

Transducer 

Beryllium copper 
6 required 



FIGURE A-l. - Three-component (3-D) borehole gauge. 



23 






O -o ® 

- 2 5 

tr = CT 



H 



^ 



c 23 






-r 



/£ 



|i 



MT 





as 



_ r __ 



LciJ 




o 



o 

CD 



< 

LL1 

O 



00 h^ CO 



V 



24 









Al 


i i 






H 1" H 
ignment pin 

Steel 
2 required 




> 


H 

5/8 


^ 


© 






:S; 


^ 







-« 2-3/4" ». 

Top 

Steel 
1 required 




««— 2" »• 

Side 

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1" 




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Transducer testing jig 




Section A-A 



6" (8") 



1/4-in stud 

Part 3 

Steel 

3 required 






r 




FIGURE A-3. - Calibration jig and transducer testing jig. 



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